1. Technical Field
The disclosure relates to a micro-electro mechanical apparatus with PN-junction.
2. Background
In recent years, due to the popularity of electronic devices such as smart phones, tablet PCs, game consoles, etc., the market of micro-electro mechanical sensors such as accelerometers and magnetometer has grown greatly. Thus, international companies have invested significant resources to develop high performance and low cost micro-electro mechanical sensors. For more and more applications of the electronic map with the function of auto orientation and the car navigation device equipped with the function of underground tunnel navigation, the new demand of the micro-electro mechanical sensors is to combine the global positioning system (GPS) with the micro-electro mechanical inertial sensors which are capable of providing direction information. Besides, to reduce the size of sensors and to keep up with the trend of being light and compact, researches of micro-electro mechanical inertial sensors tend to study how to integrate the accelerometer and the magnetometer into single sensor (i.e., multiple-axis sensing technique by using a same proof mass). In addition, it is an important issue to prevent the measured signals on different axis from interfering as new integrated micro-electro mechanical inertial sensors are developed.
FIG. 1 is the schematic diagram of a simplified X-axis micro-electro mechanical magnetometer. FIG. 2 is the schematic diagram of a simplified Z-axis micro-electro mechanical accelerometer. According to FIG. 1, the simplified X-axis micro-electro mechanical magnetometer 10 includes a movable mass 11, torsional beams 12, a coil 13, an electrode 14 and a substrate 15. The movable mass 11 is suspended above the substrate 15 and the electrode 14 by the torsional beams 12. The movable mass 11 is suitable for rotating about an axis along the torsional beam 12. In working status, an alternating current is inputted into the coil 13, wherein the current frequency is in consistent with the natural frequency of the movable mass 11. When there is a magnetic field B, Lorentz Force F is induced by the coil 13 and the magnetic field B, which drives the movable mass 11 to rotate. The magnitude of the Lorentz Force F can be calculated by the following equation:F=I·({right arrow over (L)}×{right arrow over (B)}),where I is the magnitude of electrical current, B is the magnitude of magnetic field, and L is an effective length of the coil 13 which is perpendicular to the orientation of the magnetic field B. Then, the magnitude of the magnetic force can be obtained by detecting the capacitance variation between the movable mass 11 and the electrode 14. Since the coil 13 is electrically coupled to the alternating current (AC) power source, the direction of the electrical current and the value of the voltage are both changed periodically with the phase. When the AC voltage is positive, the electrical current flows into the coil 13 from an end which is electrically coupled to the AC power source. The electrical current passes through the coil 13 on the movable mass 11, and flows into another end of the coil 13 which is electrically coupled to the ground GND. The potential difference between the coil 13 on the movable mass 11 and the substrate 15 causes accumulation of positive charges on the bottom surface of the movable mass 11, and causes accumulation of negative charges on the electrode 14 of the substrate 15. The amount of charges accumulated on the bottom surface of the movable mass 11 and the electrode 14 is accompanied with increase or decrease of the AC voltage. As the phase of the AC voltage changes (in a phase angle of 180°), and the AC voltage becomes a negative voltage, the electrical current flows from the end of the coil 13 which is electrically coupled to the ground to the end of the coil 13 which is electrically coupled to the AC power source. Owing to the change of the AC voltage, the electric potential on the bottom surface of the movable mass 11 is lower than that on the substrate 15, whereby the positive charges accumulate on the electrode 14 of the substrate 15, and the negative charges accumulate on the bottom surface of the movable mass 11. At the moment that the AC voltage alters, the electric charges on the bottom surface of the movable mass 11 and the electric charges on the electrode 14 of the substrate 15 decreases, and then an electrical current is induced on the electrode 14. The electrical current caused by changing of the amount of charges belongs to an induced current, which causes variation of voltage signal outputted from the readout circuit and makes the X-axis micro-electro mechanical magnetometer 10 to obtain an abnormal magnitude of the magnetic force.
Referring to FIG. 2, the simplified Z-axis micro-electro mechanical accelerometer 20 comprises a movable mass 21, torsional beams 22, 23, an anchor (not shown) and an electrode 24. The position of the torsional beam 22 deviates the central line of the movable mass 21, which is configured to rotate about an axis. When acceleration Az along Z axis is applied to the micro-electro mechanical accelerometer 20, the movable mass 21 rotates about the axis. Afterwards, the value of the acceleration Az along Z axis can be obtained by detecting the capacitance variation between the movable mass 21 and the electrode 24.
However, the sensing signals of magnetic force and acceleration are coupled when a common proof mass is used to sensing the magnetic force and accelerometer simultaneously. In other words, the sensing signals of magnetic force and the sensing signals of acceleration are coupled with each other. Therefore, the sensing signals needs to be decoupled to obtain the magnitude of magnetic force and the magnitude of acceleration respectively, which increases the complexity of signal processing.
Furthermore, FIG. 3A illustrates a sensor for measuring magnetic force and acceleration simultaneously of a US patent publication 2004/0158439. FIG. 3B is a cross-sectional view of the sensor along line A-A′ of FIG. 3A. According to FIGS. 3A and 3B, the sensor of FIG. 3A comprises a first movable structure 31 and a second movable structure 32, which respectively comprises a spring 31a and a spring 32a for supporting the mass 31b and mass 32b. The first and the second movable structures 31, 32 are arranged in parallel on the X-Y plane. And, electrical currents in different directions are applied to the mass 31b and mass 32b respectively. In the same magnetic field (e.g., the magnetic field Bx on X direction), the mass 31b and mass 32b move oppositely with the displacements +b and −b respectively. When an acceleration Az is applied, the two mass move in the same direction with the displacements −a and −a respectively. Therefore, the total displacement of the mass 31b is −a+b, and the total displacement of the mass 32b is −a−b, in the circumstance where the magnetic field and the acceleration both exist. The displacement caused by the magnetic field and the displacement caused by the acceleration can be respectively obtained by displacement relations of the two mass, and then magnitude of the magnetic force and the magnitude of acceleration are obtained.